Pipe extrusion die

ABSTRACT

A pipe molding die of the present invention is capable of rectifying a flow of a molten resin by a resin reservoir and uniformizing the flow of the resin extruded from the die in whichever position in a peripheral direction on a flow path within the die. Consequently, ununiformity in wall thickness of the resin pipe to be molded can be eliminated. The pipe molding die comprises a throttle part as one of components thereof, which includes a core, a shell part fitted to the core and a resin reservoir as a portion of a flow path which is formed between the core and the shell part. A molten, resin as a pipe raw material flows through the flow path. The resin reservoir is provided in at least one of the core and the shell part, and takes a ring-like shape circumscribing a central axis of the pipe molding die with the central axis centered. The resin reservoir assumes a recessed shape in cross-section.

TECHNICAL FIELD

The present invention relates to a pipe molding die and a resin pipemolded by the pipe molding die.

BACKGROUND ART

A pipe molding die is employed for manufacturing an elongate resin pipesuch as a polypropylene pipe and a polyethylene pipe that are used as,e.g., gas pipes.

The pipe molding die is supplied, from a resin extruder, with a moltenkneaded resin (the molten kneaded resin will hereinafter be termed a“molten resin”) defined as a raw material for the resin pipe. Thesupplied molten resin is discharged finally in the form of a resin pipeas an extrusion molded product from the die via a flow path within thedie. According to procedures thereof, the molten resin supplied into thedie is temporarily expanded in a cylindrical shape, and thereaftergradually throttled down into a pipe in the end that has a diametercorresponding to an application.

In general, the pipe molding die is basically constructed of a die part,a throttle part and a land part. The die part forms the molten resinsupplied in from a resin extruder in a cylindrical shape. The throttlepart gives a rectifying effect by throttling the cylindrical moltenresin fed in from the die part. And the land part uniforms a flowvelocity of the resin. Then, those constructive parts are eachconcentrically arranged in sequence from an upstream side to adownstream side in a flowing direction of the molten resin.

The resin pipe as an extrusion molded product manufactured by the pipemolding die described above is required to have no ununiformity in termsof wall thickness. Namely, it is required that both of an inner surfaceconfiguration and an outer surface configuration of the resin pipe beconcentrically complete rounds as viewed in cross-section.

It is because problems as shown in the following items (1)-(3) mightarise if the pipe has the ununiformity in wall thickness.

(1) It is undesirable in terms of external appearance.

(2) A core deviation tends to occur when in a butt seam fusion to fuseend surfaces of the pipe by butting them with each other.

(3) Contaminations and flaw on the surface of the pipe are undesirablefor joining a Joint to the pipe by fusion, and therefore the pipesurface is required to be cut. In that case, an outer peripheral surfaceof the pipe is fixed by a jig, and the pipe surface is cut by a cuttingtool while moving the cutting tool along the pipe. If ununiformity inwall thickness is large, however, the pipe cannot be uniformly held bythe jig because of the ruggedness on t he pipe surface, with the resultthat there might be a firmly fixed portion and a slackened portion tomake the pipe unstable. Further, since a distance between the outersurface of the pipe fixed by the jig and the cutting tool is notuniform, an adhesion is poor, and a complete round cannot be obtainedeven when cut off. Besides, unevenness in cutting is to appear. Acutting quantity must increase in order to-prevent the unevenness incutting, and correspondingly extra pipe raw material is needed.

Such being the case, it is a general practice that a flow of the resinextruded from the die is kept constantly in whichever position on a flowpath within the die to uniformize the wall thickness of the resin pipeto be molded by the pipe molding die. Methods of enhancing a rectifyingeffect and a throttle effect are effective in terms of keepingconstantly the flow of the resin through the flow path.

For making an attempt to enhance the rectifying effect and the throttleeffect as well, the die must be increased in size. When increasing thesize of the die, a pressure necessary for flowing the molten resin hasto be risen. Furthermore, if the pressure rises, a temperature of themolten resin increases enough to easily deteriorate the resin or tocause an excessive luster on the pipe surface to such an extent as to bevisually undesirable, resulting in a devaluation of a commercialproduct. Then, pressure tightness of the die and of the extruder must beincreased.

Moreover, according to the tests by the present inventors, it has provedthat the ununiformity in wall thickness is to occur even when making anendeavor to enhance the rectifying effect and the throttle effect in thetechnologies contrived so far in the case of manufacturing a pipe thatis equal to or larger than 8 mm in wall thickness.

It is an object of the present invention to provide a pipe molding diecapable of simply preventing an occurrence of ununiformity in wallthickness of a resin pipe irrespective of a degree of desired dimensionof the wall thickness of the resin pipe, and also a resin pipe molded bythis pipe molding die.

SUMMARYOF THE INVENTION

A pipe molding die according to the present invention comprises athrottle part defined as one the constructive parts thereof. Thisthrottle part includes a core, a shell part fitted to the core, and aresin reservoir as a portion of a flow path. The flow path is formedbetween the core and the shell part. A molten resin as a pipe rawmaterial flows through the flow path.

The resin reservoir is provided in at least one of the core and theshell part and takes a ring-like shape circumscribing a central axis ofthe pipe molding die with the central axis centered. Further, the resinreservoir assumes a recessed shape in cross-section.

The thus constructed pipe molding die according to th e presentinvention, a flow of the resin extruded from the die can be uniformizedin whichever position on the flow path within the die owing to the resinreservoir, and it is therefore feasible to restrain a momentum of theflow of the molten resin. Consequently, the flow becomes smooth toenhance a rectifying effect. Accordingly, no ununiformity in wallthickness of the resin pipe to be molded can be seen.

Moreover, a capacity of the resin reservoir may be varied correspondingto a dimension of desired wall thickness of the molded resin pipe, i.e.,the resin reservoir may be so formed as to decrease the capacity thereofin the case of a thin resin pipe but increase the capacity thereof inthe case of a thick resin pipe. A quantity of the molten resin in alongitudinal direction (The longitudinal direction means from anupstream side to a downstream side of the flow path.) at the throttlepart is thereby kept constant regardless of a degree of dimension ofdesired wall thickness of the resin pipe. The keeping constant of thequantity of the molten resin at the throttle part makes it possible toprevent an occurrence of the ununiformity in wall thickness of the resinpipe.

Thus, a size of the resin reservoir provided in the throttle part as oneof the constructive parts of the die, is simply set corresponding to thewall thickness of the resin pipe to be molded, whereby the rectifyingeffect can be enhanced without increasing the size of the die itself.

The flow path described above may include a throttle taking aconstricted shape narrower than other parts along this flow path, andthis throttle may be formed with the resin reservoir described above.

Moreover, it is desired that the resin reservoir be formed in such aconfiguration as not to cause a stagnation and a residence (thestagnation and the residence are hereinafter generically termed a“stagnation”) in the flow of the molten resin. It is desirable that arecess cross section of the resin reservoir is formed for example in acurved-surface configuration, especially a semi-circular configuration.In that case, it is preferable that a radius of curvature be 10 mm-100mm, and an angle made by the central axis and each of tangential linesat both ends of a semi-circular arc of the resin reservoir be 15°-120°.

Further, it is more desirable that the radius of curvature be 25 mm, andthe angle be 75°-90°.

The resin pipe according to the present invention is molded by using thepipe molding die as well as being molded of polyolefine as a pipe rawmaterial.

Polyolefine as the pipe raw material is desirably polyethylene.

Furthermore, it is preferable that the resin pipe be manufactured sothat an average wall thickness thereof is set to one of values in arange of 5 mm-50 mm, and that a difference between a maximum wallthickness and a minimum wall thickness of the pipe is equal to orsmaller than 1.0 mm and, preferably, equal to or smaller than 0.3 mm.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 is a vertical sectional view of a pipe molding die according tothe present invention;

FIG. 2 is an enlarged view of the principal portion in FIG. 1;

FIG. 3 is an enlarged view of the principal portion in FIG. 2;

FIG. 4 is an example of variation of that shown in FIG. 2, in which aportion for forming a resin reservoir is different;

FIG. 5 is another example of variation of that shown in FIG. 2, in whichthe portion for forming the resin reservoir is different;

FIG. 6 is a view showing a comparative example with a resin reservoiraccording to the present invention;

FIG. 7 is a view showing another comparative example with the resinreservoir according to the present invention;

FIG. 8 is a diagram showing a distribution of wall thickness of apolyethylene pipe molded by a pipe molding die according to the presentinvention;

FIG. 9 is a diagram of comparison with FIG. 8, showing a distribution ofwall thickness of a polyethylene pipe molded by a prior art pipe moldingdie; and

FIG. 10 is a view illustrating a hitherto-existing throttle portionhaving no resin reservoir, and corresponding to FIGS. 3, 6 and 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will hereinafter bedescribed with reference to the drawings.

FIG. 1 is a vertical sectional view showing one example of a pipemolding die.

In this pipe molding die 1, a molten resin is supplied from a left endin FIG. 1, and similarly a resin pipe P having a desired diameter as anextrusion molded product, is discharged from the right end. Morespecifically, the left end side of FIG. 1 corresponds to an upstreamside in a flowing direction of a molten resin p, while the right endside of the same corresponds to a downstream side. Hereinbelow, theupstream and downstream sides in the flowing direction of the moltenresin p are simply termed an “upstream side” and a “downstream side”.

The die 1 is constructed roughly of three parts. Briefly, they are a diepart 2, a throttle part 3 and a land part 4. These parts are arranged insequence from the upstream side to the downstream side of the moltenresin p. Inside the individual parts 2, 3 and 4, consecutive flow pathswhich will be mentioned later on are provided respectively. Then, themolten resin p flows sequentially through those flow paths.

The die part 2 is a part located on the upstream side of the die 1 andis a part for receiving the molten resin p supplied from anunillustrated resin extruder. The molten resin p supplied to the diepart 2 passes through the die part 2 and arrives at the throttle part 3.

Further, the die part 2 consists of a core 2 a and a shell part 2 b intowhich the core 2 a is inset.

The core 2 a is located on a central axis L of the die 1 and is acylindrical body, an upstream-side end portion of which is conicallypointed. Note that the throttle part 3 and the land part 4 respectivelyhave a core 3 a and a core 4 a, which are also located on the centralaxis L.

The shell part 2 b, which takes a cylindrical shape on the whole withone end opened and the other end closed, includes a disk-like proximalseat member 2 b ₁ located at an upstream-side end and an externalcylindrical portion 2 b ₂ occupying other portions of the shell part 2b. The external cylindrical portion 2 b ₂ is a hollow cylindrical memberextending from a peripheral edge of the proximal seat member 2 b ₁toward the downstream side.

Further, the proximal seat member 2 b ₁ has such a flat recessed portionas being hollowed out in a conical shape at the center thereof enough toreceive the above upstream side end of the core 2 a, and the externalcylindrical portion 2 b ₂ covers the cylindrical portion of the core 2a.

A flow path 2 c and a flow path 2 d of the die part 2 are respectivelyformed between the core 2 a and the proximal seat member 2 b ₁ andbetween the core 2 a and the external cylindrical portion 2 b ₂.

The flow path 2 c looks like a generally trigonal pyramid shape as thewhole (The flow path 2 c appears a forked shape gradually expanding asit approaches the downstream side in FIG. 1.). Then, an apex side of theflow path 2 c, which is located on the upstream side, is opened. Notethat the aperture of the flow path 2 c serves as a molten resinreceiving port for receiving the molten resin into the pipe molding die1 from the resin extruder, and is designated by the numeral 7.

The flow path 2 d is continued with the right side of the flow path 2 c,and takes a cylindrical configuration on the whole. In FIG. 1, the flowpath 2 d appears to be two lines of parallel passageways continuous fromthe forked flow path 2 c. A diameter of the flow path 2 d is larger thana diameter of the resin pipe P, and a screw groove 8 is formed in aninternal peripheral portion of the flow path 2 d.

The molten resin p entering the flow path 2 d from the flow path 2 cgoes on flowing toward the throttle part 3 while changing its flowingshape into a cylindrical shape from a triangular conical shape as theflow thereof advances.

Note that the die part 2 has the screw groove 8 as stated above, andhence the die 1 is termed a spiral die. The die is classified, inaddition to this, into a spider die, a cross head die, a basket die andother types of dies. In the great majority of cases, the spiral die isused, however, the variety of dies are separately employed according tothe applications and necessities.

The throttle part 3 is a part located between the die part 2 and theland part 4, and is a part for giving a so-called rectifying effect.More specifically, as already mentioned, the throttle part 3 is a partwhich admits a passage of the molten resin p flowing in from the flowpath 2 d in the cylindrical shape on the whole while being temporarilyexpanded larger in terms of its diameter than the resin pipe P, andgradually throttles the diameter of the cylindrical molten resin p downto the diameter of the resin pipe P.

The throttle part 3 described above is constructed of a core 3 a and ashell part 3 b fitted to an outer portion of the core 3 a.

The core 3 a comprises a head-cut conical part 3 a ₁ and a cylindricalpart 3 a ₂. The head-cut conical part 3 a ₁ occupies a half on theupstream side and taking a head-cut conical shape, and a cylindricalpart 3 a ₂ similarly occupies a half on the downstream side and taking acylindrical shape.

The shell part 3 b has a fitting seat member 3 b, and a throttle element3 b ₂. The fitting seat member 3 b ₁-1 corresponds to the head-cutconical part 3 a, of the core 3 a, and the throttle element 3 b ₂corresponds to the cylindrical part 3 a ₂ of the core 3 a, respectively.

The fitting seat member 3 b ₁ assumes a channel-like configuration incross-section, and a side wall part 3 b ₁-1 located on the upstream sidethereof is formed with a hole 3 b ₁-2 into which the head-cut conicalpart 3 a ₁ is inserted, taking the same configuration as this. Further,the cylindrical part 3 a ₂ of the core 3 a is located on the centralaxis L in a space 10 surrounded by a peripheral wall part 3 b ₁-3 of thefitting seat member 3 b ₁. Then, the ring-like throttle element 3 b ₂ isfitted to an outer periphery of this cylindrical part 3 a ₂.

When the core 3 a and the throttle element 3 b ₂ are fitted to thefitting seat member 3 b ₁, a flow path 3 c is formed between the hole 3b ₁-2 of the side wall part 3 b ₁-1 of the fitting seat member 3 b ₁ andthe head-cut conical part 3 a ₁ of the core 3 a, and a flow path 3 d isformed between the throttle element 3 b ₂ and the cylindrical part 3 a ₂of the core 3 a.

The flow path 3 c which is a flow path is continued with the right sideof the flow path 2 d of the die part 2 and formed a head-cut conicalshape on the whole, and the head-cut side thereof is directed toward thedownstream side. The flow path 3 c appears to be two lines ofpassageways narrowing down on the downstream side in FIG. 1.

The flow path 3 d is continued with the right side of the flow path 3 cand assumes the cylindrical shape on the whole. Further, the flow path 3d is smaller in diameter than the flow path 2 d similarly taking thecylindrical shape in the die part 2. The flow path 3 d appears as twolines of parallel passageways continuous from the flow path 3 c in FIG.1. The flow path 3 d is wide both at the upstream end and at thedownstream end thereof, but is constricted narrowly at the centralportion. This constricted part is referred to as a throttle designatedby the numeral 11. The throttle 11 is formed in such a manner that thecentral part of an inner peripheral surface 3 b ₂-1 of the throttleelement 3 b ₂ is protruded in a trapezoidal shape on the side of thecentral axis L.

The throttle 11 is, as obvious from FIG. 3, provided with a resinreservoir 13 serving as a portion of the flow path 3 d at the centralportion thereof. The throttle part 11 includes a first portion precedingthe resin reservoir 13 which constricts the flow path 3 d, and a secondportion following the resin reservoir 13 where the flow path 3 d isexpanded. The molten resin flows along the flow path 3 d and through thereservoir 13 in a direction substantially parallel to the centrallongitudinal axis L of said pipe molding die 1. The resin reservoir 13,which is a recessed portion provided in the throttle 11 and opened onthe side of the central axis L, has a ring-like annular shape about thecentral axis L. The resin reservoir 13 extends around the shell part 3 bin a direction perpendicular to the central axis L. Further, the resinreservoir 13, as obvious from FIGS. 2 and 3, is semi-circular incross-section and is 25 mm in curvature radius R. The curvature radius Ris not, however, limited to 25 mm and, though an acceptable range may be10 mm-100 mm, desirably falls within a range of preferably 10 mm-50 mmin terms of enhancing the rectifying effect and preventing so-calledblack burning by decreasing a residence time of the molten resin withinthe resin reservoir 13.

The throttle part 11 includes a first constricting portion 13 a (seeFIG. 3) immediately preceding the resin reservoir 13 which constrictsthe flow path 3 d. FIGS. 1, 2 and 4-7 show variations (not numbered) ofthe first constricting portion 13 a. The first constricting portion 13 acircumscribes the central longitudinal axis L of the pipe molding die 1.The first constricting portion 13 a has a continuous annular shape ofconstant diameter around an entire circumference thereof. Molten resinflows past the first constricting portion 13 a in a directionsubstantially parallel to the central longitudinal axis L of the pipemolding die 1. The throttle part 11 further includes a secondconstricting portion 13 b (see FIG. 3) immediately following the resinreservoir 13 which constricts the flow path 3 d. FIGS. 1, 2 and 4-7 showvariations (not numbered) of the second constricting portion 13 b . Thesecond constricting portion 13 b circumscribes the central longitudinalaxis L of the pipe molding die 1. The second constricting portion 13 bhas a continuous annular shape of constant diameter around an entirecircumference thereof. Molten resin flows past the second constrictingportion 13 b in a direction substantially parallel to the centrallongitudinal axis L of the pipe molding die 1.

Also, as shown in FIG. 3, and set within a range of 75-90°; is an angleα made by a tangential line t drawn at the upstream-side end 13 a and atthe downstream-side end 13 b of the resin reservoir 13 (only onetangential line drawn at the upstream-side end 13 a is shown). In otherwords, drawn at the two ends 13 a, 13 b of a semi-circular arc 13 c incross-section of the resin reservoir 13, and by the central axis L (inother words, a wall surface 3 a ₂′ of the cylindrical part 3 a ₂parallel to the central axis L). The angle α is not, however, confinedto the range of 75-90° and may fall within a range of 15°-120°. Inshort, this range may be the one enough to enhance the rectifying effectby the resin reservoir 13, not to cause a stagnation and to prevent theblack-burning, and preferably the one of 75-90° in terms of the effect.The numerical values given above are calculated based on an endorsementthrough the tests implemented by the present inventors.

Note that the angle α is shown in the Figures in the two cases of itsbeing made by the tangential line t and the central axis L, and by thetangential line t and the wall surface 3 a ₂′ of the cylindrical part 3a ₂ parallel to the central axis L.

Moreover, the two ends 13 a, 13 b of the circular arc are so formed asto be curvilinearly bent enough not to hinder a small influx of themolten resin. Then, gaps “a”, “b” between the two ends 13 a, 13 b andthe cylindrical part 3 a ₂ are set to 2 mm. The gaps are not, however,limited to 2 mm and are, though an acceptable range may be 0.5 mm-5 mm,desirably set to a range of preferably 1 mm-3 mm.

Then, the resin reservoir 13 is not provided in the fitting seat member3 b ₁ but may be, as illustrated in FIG. 4, provided in the cylindricalpart 3 a ₂ of the core 3 a. Further, as shown in FIG. 5, the resinreservoir 13 may be provided in both of those parts.

Moreover, the throttle 11 formed with the resin reservoir 13 may beprovided at the cylindrical part 3 a ₂ of the core 3 a and may beprovided both at the throttle element 3 b ₂ and at the cylindrical part3 a ₂.

Further, the cross-sectional shape of the resin reservoir 13 may be, inaddition to the semi-circular shape, shapes of smoothly curved surfacessuch as circular arcs in other forms, a part of elliptical shapeparabolic shape and so forth causing no stagnation of the flow of themolten resin p. However, the semicircular shape in cross section is thebest in terms of causing no stagnation of the flow of the molten resin,and is easy to work.

The resin reservoir 13 may be formed in other places than the throttle11 in the flow path 3 d.

Note that FIGS. 6 and 7 show a comparative examples with the resinreservoir 13 according to the present invention. If the resin reservoir13 takes a rectangular shape in cross-section with corners rounded asillustrated in FIG. 6 or an isosceles triangular shape in cross-sectionwith an apex rounded as shown in FIG. 7, it might happen that the moltenresin is stagnated at the corners and the apex thereof. As the result,the resin is burned black and the burned substances are adhered to thecorners and the apex as well. Accordingly, it is of importance how theconfiguration of the resin reservoir 13 is selected.

The land part 4 is a part, located on the downstream side of the die 1,for uniformizing a flow velocity of the molten resin.

Such land part 4 is constructed of a core 4 a and a shell part 4 bfitted to an outer portion of the core 4 a.

The core 4 a has a configuration similar to the core 3 a of the throttlepart 3, and is constructed of a head-cut conical part 4 a ₁corresponding to the head-cut conical part 3 a ₁ and a cylindrical part4 a ₂ corresponding to the cylindrical part 3 a ₂ of the core 3 a. Thehead-cut conical part 4 a ₁ is, however, by far smaller in differencebetween the upstream side and the downstream side than in the head-cutconical part 3 a ₁. Further, the head-cut conical part 4 a ₁ is hollow.

The shell part 4 b includes a flange member and takes a cylindricalshape on the whole. The shell part 4 b comprises a flange part 4 b ₁having the same major diameter as that of the throttle element 3 b ₂ ofthe core 3 a and being contiguous to the throttle element 3 b ₂, and anouter cylindrical part 4 b ₂ extending from a portion, closer to thecentral axis L, of the flange part 4 b ₁ toward the downstream side.

A flow path 4 c and a flow path 4 d of the land part 4 are respectivelyformed between the head-cut conical part 4 a ₁ of the core 4 a and theflange part 4 b, of the outer shell part 4 b, and between thecylindrical part 4 a ₂ of the core 4 a and the outer cylindrical part 4b ₂ of the outer shell part 4 b.

The flow path 4 c is a flow path continuous on the right side of theflow path 3 d of the throttle part 3 and takes an extremely gently slanthead-cut conical shape, and a head-cut side thereof is directedrightward in FIG. 1. The flow path 4 c appears to be two lines ofpassageways in which a spacing therebetween is narrowed down extremelygently from the flow path 3 d of the throttle part 3 as it approachestoward the downstream side in FIG. 1.

The flow path 4 d is continued with the right side of the flow path 4 cand assumes a cylindrical configuration on the whole. The flow path 4 dis smaller in diameter than the flow path 3 d similarly taking thecylindrical shape. The flow path 4 d appears two lines of parallelpassageways continuous from the flow path 4 c in FIG. 1, wherein a widthdimension “w” of each of the passages appearing parallel is so set as tobe a wall thickness of the resin pipe P defined as an extrusion moldedproduct. A diametrical dimension “W” of the flow path 4 d is, i.e., adiametrical dimension of the resin pipe P.

Incidentally, what is indicated by the numeral 15 is an outlet of theflow path 4 d, in other words, a pipe discharge port of the die 1, fromwhich the resin pipe P defined as the extrusion molded product isfinally discharged.

According to the thus constructed die 1, when the molten resin p fromthe resin extruder is supplied into the die 1 through a molten resinreceiving port 7, this molten resin p is led inwardly of the die 1 alonga route such as flow path 2 c→flow path 2 d→flow path 3 c→flow path 3d→flow path 4 c→flow path 4 d, and thereafter discharged, in the form ofthe resin pipe P as the extrusion molded product, out of the pipedischarge port 15 of the die 1.

FIG. 8 shows a distribution of wall thickness of a polyethylene pipemolded by the pipe molding die according to the present invention. Itcan be understood from FIG. 8 that a difference between a maximum wallthickness and a minimum wall thickness of the pipe, viz., anununiformity in wall thickness is extremely equal to or smaller than 0.3mm, and therefore a neat circle is depicted.

Set conditions in this case are as follows:

Curvature radius R=25 mm

α=75-90°

a, b=2 mm

Nominal dimension=200 mm (major diameter: 216 mmφ, and average wallthickness: 17 mm).

FIG. 9 is a diagram compared with FIG. 8, and shows a distribution ofwall thickness of the polyethylene pipe molded by the pipe molding diein the prior art, wherein the polyethylene pipe having the same nominaldimension of 200 as the one described above is manufactured by the diewith the throttle 11 including no resin reservoir 13 as shown in FIG.10. As can be understood from FIG. 9, the difference between the maximumwall thickness and the minimum wall thickness of the pipe, i.e., theununiformity in wall thickness is 1.6 mm, and ruggedness on a pipesurface (a pipe internal surface) can be seen.

Thus, the die 1 according to the present invention includes the resinreservoir 13, whereby there could be obtained the-polyethylene pipe witha remarkably reduced ununiformity in wall thickness, which pipe issubstantially completely round in cross-section. Note that the pipe isnot confined to the polyethylene pipe using polyethylene as a pipe rawmaterial but may embrace a polyolefine pipe using polyolefine as a piperaw material. The die 1 according to the present invention is, however,optimal to the molding of the polyethylene pipe.

As a result of generalizing the tests performed by the presentinventors, the major diameter (diameter) and the average wall thicknessof the resin pipe P formed by the pipe molding die 1 applied thereto areset respectively within a range of 60 mm-500 mm and a range of 5 mm-50mm, however, it could be recognized that the resin pipe becomespreferable by setting the major diameter of the resin pipe P within arange of 80 mm-220 mm and the average wall thickness of the pipe Pwithin a range of 8 mm-20 mm.

Then, in the distribution of wall thickness of the resin pipe P, thewall thickness ununiformity conceived as the difference between themaximum wall thickness and the minimum wall thickness can be set withina range of 0 mm-1.0 mm in the normal setting described above and a rangeof 0 mm-0.3 mm in the preferable setting described above, and thereforeit proved that the preferable resin pipe can be manufactured.

When the resin pipe P is thus manufactured by use of the die 1 accordingto the present invention, the resin p extruded from the die 1 can beflowed uniformly in whichever position on the flow path 3 d within thedie 1 owing to the resin reservoir 13, and it is therefore feasible torestrain a momentum of the flow of the molten resin p. Consequently, theflow of the molten resin p gets smooth enough to enhance the rectifyingeffect. Accordingly, it is possible to prevent the occurrence of theununiformity in wall thickness of the resin pipe P to be molded, andalso it is possible to obtain the resin pipe P with the minor and majordiameters that are both substantially completely round.

Incidentally, it can be expected that the above effect is enhanced allthe more in combination with the throttle effect.

Further, the ununiformity in wall thickness can be a,reduced, and hencethe problems described in the items (1)-(3) in the description of theprior art can be obviated.

As discussed above, in the pipe molding die according to the presentinvention, the flow of the molten resin is rectified by the resinreservoir, and the flow of the resin extruded from the die can be madeuniform in whichever position in the peripheral direction on the flowpath within the die. Therefore, the pipe molding die is applicable asthe one capable of preventing the ununiformity in wall thickness of theresin pipe to be molded. Further, the resin pipe manufactured by usingthis pipe molding die has no ununiformity in wall thickness, andtherefore a utility value thereof becomes higher correspondingly.

What is claimed is:
 1. A pipe molding die comprising: a throttle partincluding: a core; a shell part, fitted to said core, for forming a flowpath through which a molten resin as a pipe raw material flows betweensaid core and said shell part; a resin reservoir provided in at leastone of said core and said shell part as a portion of said flow path,said resin reservoir having a recessed shape in cross-section which isformed in a curved-surface configuration and a ring-like shapecircumscribing a central axis of said pipe molding die with this centralaxis centered, said flow path being substantially perpendicular to saidresin reservoir; a first constricting portion immediately preceding saidresin reservoir which constricts said flow path, said first constrictingportion circumscribing said central axis of said pipe molding die, saidfirst constricting portion having a continuous annular shape of aconstant first diameter around an entire circumference of said firstconstricting portion; a first straight portion preceding said firstconstricting portion and circumscribing said central axis of said pipemolding die, said first straight portion having a second diameter; afirst tapering portion immediately preceding said first constrictingportion and interconnecting said first straight portion to said firstconstricting portion, said first tapering portion having a diameterwhich varies between said first diameter and said second diameter; asecond constricting portion immediately following said resin reservoirwhich constricts said flow path, said second constricting portioncircumscribing said central axis of said pipe molding die, said secondconstricting portion having a continuous annular shape of a constantthird diameter around an entire circumference of said secondconstricting portion; a second straight portion following said secondconstricting portion and circumscribing said central axis of said pipemolding die, said second straight portion having a fourth diameter; anda second tapering portion immediately following said second constrictingportion and interconnecting said second constricting portion to saidsecond straight portion, said second tapering portion having a diameterwhich varies between said third diameter and said fourth diameter,wherein said molten resin flows sequentially past said first straightportion, said first tapering portion, said first constricting portion,said resin reservoir, said second constricting portion, said secondtapering portion, and said second straight portion in a directionsubstantially parallel to said central axis of said pipe molding die. 2.The pipe molding die according to claim 1, wherein said flow path has athrottle in a shape constricted thinner than other portions along saidflow path, and said throttle is formed with said resin reservoir.
 3. Thepipe molding die according to claim 1, wherein said resin reservoir issemi-circular in cross-section, a radius of curvature thereof is 10mm-100 mm, and an angle made by each of tangential lines at both ends ofthe semi-circular arc of said resin reservoir and by the central axis is15°-120°.
 4. The pipe molding die according to claim 3, wherein theradius of curvature is 25 mm, and the angle is 75°-90°.
 5. The pipemolding die according to claim 1, wherein said resin reservoir isprovided in both said core and said shell part.
 6. The pipe molding dieaccording to claim 1, wherein said resin reservoir extends around saidshell par in a direction perpendicular to said central axis.
 7. A pipemolding die comprising: a throttle part including: a core; a shell part,fitted to said core, for forming a flow path through which a moltenresin as a pipe raw material flows between said core and said shellpart; a resin reservoir provided in at least one of said core and saidshell part as a portion of said flow path, said resin reservoir having arecessed shape in cross-section which is formed in a curved-surfaceconfiguration and a continuous annular shape circumscribing a centrallongitudinal axis of said pipe molding die, said flow path beingsubstantially perpendicular to said resin reservoir; a firstconstricting portion immediately preceding said resin reservoir whichconstricts said flow path, said first constricting portioncircumscribing said central longitudinal axis of said pipe molding die,said first constricting portion having a continuous annular shape of aconstant first diameter around an entire circumference of said firstconstricting portion; a first straight portion preceding said firstconstricting portion and circumscribing said central axis of said pipemolding die, said first straight portion having a second diameter; afirst tapering portion immediately preceding said first constrictingportion and interconnecting said first straight portion to said firstconstricting portion, said first tapering portion having a diameterwhich varies between said first diameter and said second diameter; asecond constricting portion immediately following said resin reservoirwhich constricts said flow path, said second constricting portioncircumscribing said central axis of said pipe molding die, said secondconstricting portion having a continuous annular shape of a constantthird diameter around an entire circumference of said secondconstricting portion; a second straight portion following said secondconstricting portion and circumscribing said central axis of said pipemolding die, said second straight portion having a fourth diameter; anda second tapering portion immediately following said second constrictingportion and interconnecting said second constricting portion to saidsecond straight portion, said second tapering portion having a diameterwhich varies between said third diameter and said fourth diameter,wherein said molten resin flows sequentially past said first straightportion, said first tapering portion, said first constricting portion,said resin reservoir, said second constricting portion, said secondtapering portion, and said second straight portion in a directionsubstantially parallel to said central longitudinal axis of said pipemolding die.
 8. The pipe molding die according to claim 7, wherein saidflow path has a throttle in a shape constricted thinner than otherportions along said flow path, and said throttle is formed with saidresin reservoir.
 9. The pipe molding die according to claim 7, whereinsaid resin reservoir is semi-circular in cross-section, a radius ofcurvature thereof is 10 mm-100 mm, and an angle made by each oftangential lines at both ends of the semi-circular arc of said resinreservoir and by the central axis is 15°-120°.
 10. The pipe molding dieaccording to claim 9, wherein the radius of curvature is 25 mm, and theangle is 75°-90°.
 11. The pipe molding die according to claim 7, whereinsaid resin reservoir is provided in both said core and said shell part.12. The pipe molding die according to claim 7, wherein the resinreservoir extends around said shell part in a direction perpendicular tosaid central axis.
 13. The pipe molding die according to claim 1,wherein said first tapering portion is conical.
 14. The pipe molding dieaccording to claim 13, wherein said second tapering portion is conical.15. The pipe molding die according to claim 1, wherein said diameter ofsaid first tapering portion varies linearly.
 16. The pipe molding dieaccording to claim 15, wherein said diameter of said second taperingportion varies linearly.
 17. The pipe molding die according to claim 7,wherein said first tapering portion is conical.
 18. The pipe molding dieaccording to claim 17, wherein said second tapering portion is conical.19. The pipe molding die according to claim 7, wherein said diameter ofsaid first tapering portion varies linearly.
 20. The pipe molding dieaccording to claim 19, wherein said diameter of said second taperingportion varies linearly.